# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved

import torch


class _InfiniteSampler(torch.utils.data.Sampler):
    """Wraps another Sampler to yield an infinite stream."""

    def __init__(self, sampler):
        self.sampler = sampler

    def __iter__(self):
        while True:
            for batch in self.sampler:
                yield batch


class InfiniteDataLoader:

    def __init__(self, dataset, weights, batch_size, num_workers):
        super().__init__()

        if weights is not None:
            sampler = torch.utils.data.WeightedRandomSampler(
                weights, replacement=True, num_samples=batch_size)
        else:
            sampler = torch.utils.data.RandomSampler(dataset, replacement=True)

        if weights == None:
            weights = torch.ones(len(dataset))

        batch_sampler = torch.utils.data.BatchSampler(sampler,
                                                      batch_size=batch_size,
                                                      drop_last=True)

        self._infinite_iterator = iter(
            torch.utils.data.DataLoader(
                dataset,
                num_workers=num_workers,
                batch_sampler=_InfiniteSampler(batch_sampler)))

    def __iter__(self):
        while True:
            yield next(self._infinite_iterator)

    def __len__(self):
        raise ValueError


class FastDataLoader:
    """DataLoader wrapper with slightly improved speed by not respawning worker
    processes at every epoch."""

    def __init__(self, dataset, batch_size, num_workers):
        super().__init__()

        batch_sampler = torch.utils.data.BatchSampler(
            torch.utils.data.RandomSampler(dataset, replacement=False),
            batch_size=batch_size,
            drop_last=False)

        self._infinite_iterator = iter(
            torch.utils.data.DataLoader(
                dataset,
                num_workers=num_workers,
                batch_sampler=_InfiniteSampler(batch_sampler)))

        self._length = len(batch_sampler)

    def __iter__(self):
        for _ in range(len(self)):
            yield next(self._infinite_iterator)

    def __len__(self):
        return self._length
